The 20-m Large Deployable Reflector (LDR) will have a segmented primary mirror. Achieving diffraction-limited performance at 50 μm requires correction for the errors of tilt and piston of the primary mirror. This correction can be obtained in two ways, use of an active primary or correction at a de-magnified pupil of the primary. A critical requirement is the means for measurement the wavefront error and maintaining phasing during observation of objects that may be too faint for determining the error. Absolute phasing can only be determined using a cooperative source. Maintenance of phasing can be done with an on-board source. A number of options are being explored as discussed below. The many issues concerning the assessment and control of an active segmented mirror will be addressed with early construction of the Precision Segmented Reflector (PSR) Testbed.